Predicting the date and time of an earthquake is impossible, but seismologists can use trends in tectonic activity to suggest where a big tremor might occur. A recent study revealed a massive, hidden fault line running under miles of river sediment. Scientists suggest the fault, which runs beneath Bangladesh, parts of east India and Myanmar, could cause a magnitude 8.2 to 9.0 earthquake. A quake of that strength would be devastating in such a densely populated region.

The recently discovered fault has created a lot of work for researchers, who are eager to learn more about it. However, since the fault has been hidden for so long, seismologists aren’t able to say as much about it as other faults around the world that have been under close watch for decades. Without knowledge of the fault’s trends over time, little can be done to protect the very people who might suffer if a major earthquake occurs along the fault line.

“We don’t know if it’s tomorrow or if it’s not going to be for another 500 years,” said study co-author Michael Steckler, a geophysicist at the Lamont-Doherty Earth Observatory (LDEO) at Columbia University in New York City.

Steckler and his colleagues discovered the fault while working to map plate-motion data throughout Bangladesh, India, and Myanmar. The team, working with researchers at Bangladesh’s Dhaka University, used ultrasensitive GPS devices throughout Bangladesh between 2003 and 2014 to collect the data, which revealed the existence of this previously unknown fault. Researchers estimate that some 140 million people live within a 60-mile radius of the fault and, because of unsustainable building practices (like pumping sand out of the ground to build up areas for skyscrapers), a major earthquake in the region would leave countless casualties and massive infrastructure damage.

The results of the study were published this week in the journal Nature Geoscience.